GMOS-S Hamamatsu

A high signal-to-noise 4x4 binned bias image for GMOS-S with the Hamamatsu CCDs is given below (overscan subtracted). These data were taken in slow read, using all twelve amplifiers. A single line plot through the bias image is also presented. As it can be seen, the bias images have some structure, in a similar way than GMOS-N CCDs has. This structure is readily visible in this high signal-to-noise example but is still present in individual bias images. There is also one bright column associated with amplifier #5.

It is recommended therefore to always create and subtract bias images from science data in order to remove the structure. In addition, overscan subtraction is recommended particularly for faint target spectroscopy as the overscan level is expected to drift with time both in absolute value and with respect to each individual amplifier. In the data presented here the average overscan levels were as follows:

GMOS-N e2v DD

A high signal-to-noise 4x4 binned bias image for GMOS-N with the interim upgrade e2vDD CCDs is given below. This image is composed from 1000 individual bias images, and has been overscan subtracted. These data were taken in slow read, using all six amps. A single line plot through the bias image is also presented. There are several notable features in this bias image:

The level is not zero, as might be expected from an overscan subtracted bias image. The average bias level after overscan subtraction is on the order of 0.6 ADU.

There is one bright column associated with every amplifier. If the amplifier is on the right side of the detector then the bright column is the furthest one to the left of the image (the last image pixel read out before the overscan). For six amp data this leads to the unfortunate result of two bright columns in the middle of each detector. The level in each column is between 1-3 ADUs. The accelerated schedule for the installation of these detectors in GMOS-N left no time to optimize the operating voltages which may have reduced or eliminated this effect.

In addition to an overall bias level and the bright columns, the bias images have additional structure. This structure is readily visible in this high signal-to-noise example but is still present in individual bias images.

There is evidence of high-frequency noise in four of the amps, particularly in CCD1 but at a lower level also in CCD3. The bias image for CCD2 show no evidence of this effect.

It is recommended therefore to always create and subtract bias images from science data in order to remove the features listed above. In addition, overscan subtraction is recommended particularly for faint target spectroscopy as the overscan level is expected to drift with time both in absolute value and with respect to each individual amplifier. In the data presented here the average overscan levels were as follows:

extension [1]: 1180

extension [2]: 1059

extension [3]: 915

extension [4]: 1106

extension [5]: 1187

extension [6]: 1105

Please remember that with this detector controller the science extensions do not correspond to the pixel data physical location in a straightforward manner. Refer to the diagram on the GMOS data format and reduction webpage for the mapping of image extension to pixel location.

Line cut through the GMOS-N bias image above. This is a high signal-to-noise image derived from 1000 individual bias frames, the read noise on a single bias image varies by amplifier between 1.35-1.45 ADUs. Please see the GMOS-N Array (e2v DD) webpages for gain and read noise information from individual amplifiers and for the different readout modes.

GMOS-S EEV blue

An example bias image for GMOS-S with the E2V CCDs is
shown below. In slow read out and using the best amplifiers the bias
levels during the commissioning observations were 599 ADU/pixel, 579
ADU/pixel and 627 ADU/pixel for detector 1, 2 and 3, respectively. The
greyscale used blow is chosen to show the spatial structure in the
bias frame, rather than the relative levels in the three
detectors. The data are from the commissioning and have been released
as part of dataset
GS-CAL20030524.

Bias image: Slow read and best amplifiers.

GMOS-N EEV red

An example bias image for GMOS-N with the original red EEV CCDs is shown below.
In slow read out and using the best amplifiers the bias levels during the commissioning
observations were 749 ADU/pixel, 633 ADU/pixel and 387 ADU/pixel for detector
1, 2 and 3, respectively. The greyscale used below is chosen to show the
spatial structure in the bias frame, rather than the relative levels in
the three detectors.
The data are from the commissioning and have been released as part of
dataset
GN-CAL20010910.